1. Field of the Invention
The present invention relates to a propeller shaft for use as a mechanical driving power transmission shaft in an automobile or the like, and a method of producing the same.
2. Description of the Prior Art
A propeller shaft of an automobile conveys the engine power from a transmission to a final reduction gear, and includes an intermediate shaft and joints disposed at both ends of the intermediate shaft. The propeller shaft has a structure adaptable to variations in the length and angle which are caused by change in the relative position between the transmission and the final reduction gear. Conventionally, the joints constituting the propeller shaft and the intermediate shaft are typically made of steel.
In view of flexural rigidity as well as being made of steel as described above, a long propeller shaft is constructed in parts and a bearing or the like is needed for supporting its intermediate portion, so that an improvement is required in terms of weight, costs, and others. Therefore, in order to meet these demands, adoption of a fiber reinforced plastics (hereafter referred to as FRP) is proposed (See, for example, Japanese Laid-open Patent Publications Nos. 53-71422, 55-118831, and 63-199914. In other words, a longer shaft can be produced by forming a hollow FRP shaft having a high flexural rigidity on an outer circumference of a metal shaft, thereby eliminating the need for division and the need for a supporting bearing in the intermediate portion. This hopefully leads to light weight and low costs.
In a proposed example, a pipe made of FRP alone having a high specific strength is adopted. However, its joint portion can hardly be integrally molded with FRP in view of the rigidity and strength. Typically, metal joints are joined to end portions of the FRP. As a joining method, there are proposed a method in which a sleeve of a metal joint is press-fitted into or bonded with an adhesive to an end portion of the FRP pipe or alternatively, after a metal joint is inserted into an FRP pipe, a continuous fiber impregnated with a resin is wound around the FRP pipe together with the joint (See Japanese Laid-open Patent Publication No. 55-118831), and a method in which a fitting portion of a metal pipe is made in a non-circular cross sectional shape, and an end portion of an FRP pipe is heated above a glass transition temperature and caulked onto a fitting portion of the joint (See Japanese Laid-open Patent Publication No. 63-199914).
As another joining method, in order to achieve torque transmission by ensuring strength of a joining portion, joining has been performed by a method allowing a shaft end to have a polygonal cross section, by performing a surface-roughening treatment with a knurl roller on a joining surface at a portion at which a shaft end portion of a hollow shaft is superposed, by caulking a hollow shaft made of FRP, or by press-fitting a metal component into a shaft core portion of a hollow shaft. Further, if an end portion of a hollow shaft made of FRP is joined to a metal component with an adhesive between the two, various measures have been taken to ensure the strength of the joining portion by utilizing both an adhesive and a process such as surface roughening, caulking, or press-fitting.
However, according to these conventional methods, there are various problems in molding, such as difficulty in processing a shaft end portion, a need for increasing the outer diameter for ensuring the strength of the joining portion, or an additional need for taking measures against slipping-off in an axial direction for ensuring reliability.
Caulking a hollow shaft made of FRP or press-fitting a metal component into a shaft core of a hollow shaft involves a serious disadvantage of lack in a long-term reliability caused by decrease in a binding force due to creep or stress relaxation at the FRP portion, leading to possible sliding in a circumferential direction or possible slipping-off in an axial direction.
Further, when one observes the joining portion, torque transmission is performed only by utilizing a contact area between the FRP and the metal component with the help of a frictional force obtained by utilizing a binding force generated at the time of press-fitting as a normal force, or chemical and physical bonding with an adhesive disposed at their interface. In this case, in an attempt to meet an impulsively generated excessive torque, an area of the interface is enlarged as much as possible by increasing the amount of press-fitting, or an elastic deformation amount by press-fitting of the FRP is increased. This leads to problems in joining, such as generation of cracks in the FRP portion during the process or unavoidable creep or stress relaxation when in use.
On the other hand, if a hollow shaft made of FRP is used as a intermediate shaft of a propeller shaft serving as a mechanical power transmission shaft for light weight, improved fuel economy, low cost, improvements in N.V.H.(Noise, Vibration and Harshness) behavior, and the like, there is a need to reduce the outer diameter of the hollow shaft owing to the limited space in the automobile.
Therefore, it is an object of the present invention to provide a propeller shaft having a high reliability in joining and a high rigidity, i.e. a high natural bending frequency, with light weight and low costs in order to meet the aforementioned need for improvements.